HST-COS SPECTROSCOPY OF THE COOLING FLOW IN
A1795—EVIDENCE FOR INEFFICIENT STAR FORMATION
IN CONDENSING INTRACLUSTER GAS
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Citation
McDonald, Michael, Joel Roediger, Sylvain Veilleux, and Steven
Ehlert. “HST-COS Spectroscopy of the Cooling Flow in
A1795—Evidence for Inefficient Star Formation in Condensing
Intracluster Gas.” The Astrophysical Journal 791, no. 2 (August
6, 2014): L30. © 2014 The American Astronomical Society
As Published
http://dx.doi.org/10.1088/2041-8205/791/2/l30
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Thu May 26 07:08:09 EDT 2016
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The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
C 2014.
doi:10.1088/2041-8205/791/2/L30
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
HST-COS SPECTROSCOPY OF THE COOLING FLOW IN A1795—EVIDENCE FOR INEFFICIENT
STAR FORMATION IN CONDENSING INTRACLUSTER GAS
1
Michael McDonald1,6 , Joel Roediger2,3 , Sylvain Veilleux4,5 , and Steven Ehlert1
Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA 02139, USA; mcdonald@space.mit.edu
2 Department of Astronomy, University of California Santa Cruz, CA 95064, USA
3 Department of Physics, Engineering Physics, & Astronomy, Queen’s University, Kingston, Ontario, Canada
4 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
5 Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
Received 2014 May 30; accepted 2014 July 15; published 2014 August 6
ABSTRACT
We present far-UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope of a cool,
star-forming filament in the core of A1795. These data, which span 1025 Å < λrest < 1700 Å, allow for the
simultaneous modeling of the young stellar populations and the intermediate-temperature (105.5 K) gas in this
filament, which is far removed (∼30 kpc) from the direct influence of the central active galactic nucleus. Using a
combination of UV absorption line indices and stellar population synthesis modeling, we find evidence for ongoing
+0.2
star formation, with the youngest stars having ages of 7.5+2.5
−2.0 Myr and metallicities of 0.4−0.1 Z . The latter is
consistent with the local metallicity of the intracluster medium. We detect the O vi λ1038 line, measuring a flux
of fO VI,1038 = 4.0 ± 0.9 × 10−17 erg s−1 cm−2 . The O vi λ1032 line is redshifted such that it is coincident with a
strong Galactic H2 absorption feature, and is not detected. The measured O vi λ1038 flux corresponds to a cooling
rate of 0.85 ± 0.2 (stat) ± 0.15 (sys) M yr−1 at ∼105.5 K, assuming that the cooling proceeds isochorically,
which is consistent with the classical X-ray luminosity-derived cooling rate in the same region. We measure a star
formation rate of 0.11 ± 0.02 M yr−1 from the UV continuum, suggesting that star formation is proceeding at
13+3
−2 % efficiency in this filament. We propose that this inefficient star formation represents a significant contribution
to the larger-scale cooling flow problem.
Key words: galaxies: clusters: individual (A1795) – galaxies: clusters: intracluster medium – galaxies: star
formation – galaxies: stellar content
Online-only material: color figures
2011; McDonald et al. 2012c), there is still very little evidence
for gas between 104 K and 107 K, which would directly link
the hot and cool phases. High-resolution X-ray spectroscopy
of individual clusters has, thus far, only been able to put upper
limits on the amount of ∼106 K gas in the cores of galaxy clusters
(e.g., Peterson et al. 2003; Peterson & Fabian 2006; Sanders
et al. 2010). Recently, Sanders & Fabian (2011) performed a
stacking analysis of X-ray grating spectra, yielding a detection
of O vii at a level ∼4–8 times lower than the expectation from
simple cooling flow models. Using the FUSE satellite, Oegerle
et al. (2001) and Bregman et al. (2001, 2006) found evidence for
O vi emission in the far-UV (FUV), probing gas at ∼105.5 K, in
the cores of several galaxy clusters, inferring cooling rates well
below the cooling flow expectation. These observations were
nearly always centered on the nucleus of the central cluster
galaxy, where AGN feedback could in fact be heating the gas,
causing excess O vi emission.
Here we present new FUV spectroscopy (Section 2) of Abell
1795 (A1795), a nearby, strongly-cooling galaxy cluster (see,
e.g., Fabian et al. 2001; Ettori et al. 2002; Ehlert et al. 2014).
These spectra were obtained along the southwestern filament,
which is observed to be cooling rapidly in the X-ray (Crawford
et al. 2005; McDonald et al. 2010; Ehlert et al. 2014), contains
warm (104 K) ionized (Cowie et al. 1983; Crawford et al. 2005;
McDonald & Veilleux 2009) and cold molecular gas (Salomé &
Combes 2004; McDonald et al. 2012c), and is rapidly forming
stars (∼1 M yr−1 ; McDonald & Veilleux 2009). With deep
FUV spectroscopy, we can estimate the age and metallicity of
these newly-formed stars (Section 3), allowing us to determine
1. INTRODUCTION
In the cores of galaxy clusters, the intracluster medium (ICM)
can reach high enough density and low enough temperature
that the inferred cooling time is much shorter than the age
of the universe. In these so-called “cooling flow clusters,”
simple models predict that ∼100–1000 M yr−1 of cool gas
should condense out of the hot intracluster plasma and fuel star
formation in the central cluster galaxy (for a review, see Fabian
1994). However, with few notable exceptions (McNamara et al.
2006; McDonald et al. 2012a), we do not observe such vigorous
starbursts at the centers of galaxy clusters: the typical star
formation rate in the core of a cooling flow cluster is only
∼1–10 M yr−1 (e.g., Hicks & Mushotzky 2005; O’Dea et al.
2008; McDonald et al. 2011). This low level of star formation is
most likely being fueled by local thermodynamic instabilities in
the ICM (e.g., McCourt et al. 2012), with the remaining 90%
of the energy lost from cooling being offset by radio-mode
feedback from the central active galactic nucleus (AGN; e.g.,
Churazov et al. 2001; Rafferty et al. 2006, 2008; Fabian 2012;
McNamara & Nulsen 2012).
While most cooling flow clusters show evidence of such
“reduced cooling flows” in the form of ongoing star formation
(e.g., Johnstone et al. 1987; McNamara & O’Connell 1989;
Allen 1995; O’Dea et al. 2008; Hicks et al. 2010; McDonald
et al. 2011) and cold molecular gas (e.g., Edge 2001; Edge et al.
2002; Salomé & Combes 2003; Hatch et al. 2005; Salomé et al.
6
Hubble Fellow.
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The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
McDonald et al.
Figure 1. Left: Continuum (∼7000 Å) image of the central galaxy in A1795. Left-center: Hα image showing the twin filaments extending ∼50 kpc to the south of the
central galaxy (McDonald & Veilleux 2009). Right-center: Far-UV image from HST ACS/SBC (McDonald & Veilleux 2009) showing young stellar populations along
the Hα filaments. Right: Zoom-in on the far-UV image, showing the location of the 2. 5 COS aperture. This aperture, which is shown in all four panels, is centered
on the brightest UV clump in the western filament. In the leftmost panel, the positions of the two closest galaxies are denoted by A and B. These galaxies are 16 and
21 kpc away in projection from the COS pointing, respectively, implying that any gas/stars in the filaments that originated in the satellite galaxies would have been
stripped from these galaxies >26 Myr ago (assuming v = 800 km s−1 ).
if they are forming in situ or have been tidally stripped, while
also measuring the O vi emission line flux (Section 4) far from
the influence of the central AGN (∼30 kpc). This approach will
allow us to link the young stars to the cooling X-ray gas, if that is
indeed their origin (Section 5). We will finish, in Section 6, with
a discussion of the current state of the cooling flow problem,
and how these new data can advance our understanding.
Throughout this Letter, we assume H0 = 70 km s−1 Mpc−1 ,
ΩM = 0.27, and ΩΛ = 0.73.
Since the M09 models do not consider contributions of other
hot stellar phases (e.g., blue horizontal branch), we implicitly
assume that the observed FUV emission is entirely due to young
(<1 Gyr) stars.
The M09 models come in two flavors: ones based on empirical fitting functions to IUE spectra of Milky Way and Magellanic Cloud (MC) stars or others based on theoretical Kurucz spectra. We use the latter here given that they reproduce
the ages of MC globular clusters (from color–magnitude diagrams) to within a mean residual of 0.02 ± 0.32 dex (see
Appendix C of M09). These models span a semi-regular grid of
ages from 1 Myr to 1 Gyr and total metallicities from −1.00 to
+0.35 dex with respect to solar.8 We allow age and metallicity
to vary simultaneously in our fits, which follow a maximum
likelihood approach. Statistical errors in the best-fit quantities
are estimated via Monte Carlo simulations of the measured index errors. Although M09 recommend fitting to all IUE indices
in the 1000–2000 Å range at once with their theoretical models, we have performed fits to a variety of index combinations,
specifically a blue set (BL1302, Si iv, BL1425) and a red set
(C iva, C iv, C ive, BL1617, BL1664), in order to test the robustness of our results. These results are summarized in Table 1.
Overall, we find consistently low ages (7.5+2.5
−2.0 Myr) and metallicities (0.4+0.2
Z
)
regardless
of
which
combination
of indices
−0.1 we employ.
2. DATA
FUV spectroscopy for this program was acquired using the
Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST) using the G140L grating with λcenter = 1280 Å,
which yields a spectral coverage of 1080–1900 Å. The 2. 5
aperture was centered at (α, δ) = 207◦.2199, +26◦ .5873, which
corresponds to the peak of both the FUV and Hα emission along
the filament (see Figure 1).
COS spectroscopy is simultaneously obtained in “blue”
and “red” channels, with respective wavelengths spanning
∼1080–1200 Å and ∼1250–1800 Å for our setup. The gap
between 1200 and 1250 Å spans the geocoronal Lyα line
(1216 Å). We observe one other strong geocoronal line due
to O i at ∼1302.2–1306 Å,7 which is redward of redshifted Lyβ
(λLyβ = 1290.8 Å). We determine the redshift of the spectrum
using a joint fit to the Lyβ emission line and two absorption
features in the red channel (Si iv λ1394 and C iv λ1548). This
fit yields z = 0.0619 ± 0.0005, which is consistent with our
optical redshift of z = 0.0618 from McDonald et al. (2012b) at
the same position along the filament.
3.2. Full Spectrum Synthesis
A complementary method of constraining the age and metallicity of the UV-emitting stellar population is to model the full
spectrum using synthetic stellar population models. For this, we
opt to use the latest version of Starburst99 (v7.0.0; Leitherer et al.
1999),9 which is, to our knowledge, the only publicly-available
stellar population synthesis code with spectral resolution better
than 1 Å over 1000 Å < λ < 2000 Å. We restrict this analysis
to >1100 Å—at bluer wavelengths, Starburst99 uses empirical
stellar spectra which provide a qualitatively-poor fit to the data
(see Section 4). We use the latest Geneva tracks, which are only
available for solar (Ekström et al. 2012) and 0.14 Z (Georgy
3. FUV CONTINUUM: YOUNG STELLAR POPULATIONS
3.1. UV Absorption Indices
To constrain the age and metallicity of the stellar population
responsible for the observed FUV continuum in A1795, we
use predicted UV absorption line strengths from Maraston
et al. (2009, hereafter M09), which are cast in terms of
the International Ultraviolet Explorer (IUE) index system
established by Fanelli et al. (1992). The use of an index-based
approach helps reduce the uncertainty in our results due to
reddening effects, which are most severe at UV wavelengths.
7
8
The spacing of the age grid changes from 0.5 Myr, to 5 Myr, to 50 Myr in
the intervals 1 Myr-10 Myr, 10 Myr-0.1 Gyr, and 0.1–1.0 Gyr, respectively. On
the other hand, the models are spaced at 0.1 dex intervals in terms of
metallicity, except the +0.35 dex models, which are offset by 0.15 dex from the
+0.20 dex models.
9 http://www.stsci.edu/science/starburst99/docs/default.htm
http://www.stsci.edu/hst/cos/calibration/airglow_table.html
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The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
McDonald et al.
Figure 2. Far-UV spectrum from the red channel of our HST-COS observation. This spectrum has been binned in wavelength by a factor of 27 (2.2 Å) in order
to improve signal-to-noise. Vertical gray bands correspond to spectral indices as defined by Fanelli et al. (1992). Overplotted are various models generated with
Starburst99 (Leitherer et al. 1999) using the latest stellar tracks from Ekström et al. (2012) and Georgy et al. (2013), demonstrating the overall quality of these fits to
the data.
(A color version of this figure is available in the online journal.)
Table 1
Stellar Age and Metallicity from UV Absorption Line Indices
Indices
BL1302, Si iv, BL1425, C iva, C iv, C ive, BL1617, BL1664
log10 (Z) [Z ]
7.5+2.5
−2.0
−0.4+0.2
−0.1
10.0+10.0
−4.5
−0.2+0.2
−0.2
BL1425, C iva, C iv, C ive, BL1617, BL1664
7.5+2.5
−2.0
−0.4+0.2
−0.3
C iva, C iv, C ive, BL1617, BL1664
7.5+2.0
−4.0
−0.4+0.2
−0.1
C iv, C ive, BL1617, BL1664
9.5+0.5
−2.0
−0.5+0.1
−0.1
C ive, BL1617, BL1664
3.0+4.0
−1.0
6.5+3.5
−3.0
7.5+2.5
−2.0
7.5+2.5
−2.0
−0.1+0.5
−0.3
Si iv, BL1425, C iva, C iv, C ive, BL1617, BL1664
BL1302,
Age [Myr]
BL1302, Si iv,
BL1302, Si iv, BL1425,
BL1302, Si iv, BL1425, C iva,
BL1302, Si iv, BL1425, C iva, C iv,
BL1617, BL1664
BL1302, Si iv, BL1425, C iva, C iv, C ive,
BL1664
BL1302, Si iv, BL1425, C iva, C iv, C ive, BL1617
−0.4+0.2
−0.1
−0.4+0.1
−0.1
−0.4+0.2
−0.1
Notes. All uncertainties are 1σ . See Maraston et al. (2009) for a discussion of far-UV absorption indices.
et al. 2013) metallicities. We explore three different initial mass
functions (IMFs): Salpeter (Salpeter 1955, α = −2.35), with
top-heavy (α = −1.35) and top-light (α = −3.35) variants. For
each choice of metallicity and IMF, we generate synthetic spectra assuming either a burst or continuous star formation, over
timescales of 50 Myr. These model spectra are fit to the data,
allowing the normalization, redshift, and reddening to vary, with
a lower limit of the Galactic value imposed on the reddening
(Schlegel et al. 1998).
Figures 2 and 3 show the results of this exercise. In Figure 2 we
compare several best-fitting models to the data, demonstrating
the overall quality of these fits. These synthetic spectra are able
to adequately fit the various absorption features discussed in
2
Section 3.1. The goodness-of-fit (χdof
) is shown in Figure 3 as a
function of age, metallicity, and IMF, for both instantaneous
and continuous star formation. All starburst models favor a
relatively young population, with the majority showing minima
at ages of 5 Myr. The continuous star formation models prefer
either a Salpeter IMF, with ages >10 Myr or a younger top-light
stellar population. As is also the case for the burst-like models,
the distinguishing power between IMFs comes from the C iv
feature (see Figure 2), which is likely contaminated by emission.
Qualitatively, all three choices of IMF perform equally well in
fitting the spectrum over the range 1220 Å < λrest < 1700 Å.
This analysis corroborates the results from FUV line indices (Section 3.1), demonstrating that the best-fitting stellar
population is one that is either currently forming stars, or ceased
very recently (10 Myr ago).
4. FUV EMISSION LINES: PROBING 105.5 K
GAS WITH O vi
In Figure 4 we show the blue side (<1200 Å) of the
spectrum. These data are significantly noisier than the red
channels, and suffer from airglow emission and Galactic H2
absorption to a higher degree. Despite this, there is evidence for
emission at redshifted O vi λ1038 in the binned spectrum (upper
panel of Figure 4). The spectrum of Galactic H2 absorption
from McCandliss (2003) shows strong absorption at the same
wavelength as redshifted O vi λ1032, which likely explains the
lack of emission from the bluer line in this doublet, which should
have a factor of 2 higher flux. At these wavelengths, Starburst99
relies on empirical stellar spectra (see Section 3) which provide
a relatively poor fit to the data, so we opt to model the continuum
spectrum by performing a median smoothing over a 2.4 Å
(∼650 km s−1 ) window, which should be significantly wider
than any emission lines. The residual spectrum with this model
subtracted is shown in the lower panel of Figure 4.
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The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
McDonald et al.
Figure 4. Upper panel: Far-UV spectrum from the blue channel of our
HST-COS observation. Here, we show the binned (0.8 Å) data along with
stellar population synthesis models from Starburst99 (Leitherer et al. 1999)
and a median continuum spectrum computed with a 2.4 Å smoothing window.
At these wavelengths Starburst99 only provides empirical models, which yield
poor fits to the data (note that the red channels are well-fit by a solar-metallicity
model; Figure 2). Lower panel: Residual spectrum, with the median continuum
level subtracted. The expected location of the O vi λλ1032,1038 doublet is
highlighted. In purple, we plot the spectrum of Galactic H2 absorption from
McCandliss (2003), which shows that the bluer O vi λ1032 line is most likely
being absorbed by molecular gas in our galaxy. The correspondence between
a saturated absorption line and the O vi λ1032 line is highlighted by a vertical
dashed line. We detect the redder, unabsorbed line at >4σ significance, as shown
in the inset, with a velocity width of 0.7 Å (90 km s−1 ).
(A color version of this figure is available in the online journal.)
2 ) as a function of age for instantaneous (upper
Figure 3. Goodness-of-fit (χdof
panel) and continuous (lower panel) star formation. For all choices of IMF and
metallicity, the data are well-represented by either a young (10 Myr) stellar
population or ongoing star formation.
(A color version of this figure is available in the online journal.)
cool gas has been stripped from nearby gas-rich galaxies and is
being heated via conduction by the hot ICM. The lack of stars in
the filaments surrounding M87 was offered as further evidence
of this scenario.
The detection of young (10 Myr) stars and cold molecular
gas (McDonald et al. 2012c) in the filaments of A1795 favors the
scenario in which gas is condensing and stars are forming in situ.
If, instead, the cool gas originated in a nearby galaxy, it would
have been stripped at least 26 Myr ago, given the distance to the
nearest satellite galaxy (21 kpc; Figure 1) and the typical galaxy
velocities in the core of A1795 (∼800 km s−1 ). Assuming the
stars are forming in situ, it seems unlikely that the filamentary
gas is simultaneously condensing (into stars) and evaporating
(causing coronal emission)—the simplest explanation is that the
hot intracluster gas is cooling through the O vi transition and into
molecular gas, before ultimately forming stars. The agreement
between the measured metallicity of the stars (0.4+0.2
−0.1 Z ;
Table 1) and the cooling ICM at the same position (0.5+0.3
−0.2 Z ;
Ehlert et al. 2014) further supports this scenario.
If we assume that the O vi λ1038 emission is due to cooling
intracluster gas, we can estimate the cooling rate following
Edgar & Chevalier (1986) and Voit et al. (1994). The inferred
cooling rate can vary by a factor of ∼1.6 depending on if the
cooling proceeds isobarically or isochorically. Based on very
deep X-ray data of the cooling filament (Ehlert et al. 2014),
we estimate the sound-crossing time at the location of the COS
Assuming a linewidth of 0.43 Å (σ = 50 km s−1 ), based
on the Hα velocity dispersion (McDonald et al. 2012b),
we measure a flux of fO vi = 4.0 ± 0.9 × 10−17 erg s−1 cm−2
in the redder line of the O vi doublet (>4σ detection). While
the statistical significance of this line is low, we note that it is
the most statistically significant deviation over the entire blue
spectrum. The fact that this deviation is at the wavelength that
we expect to find redshifted O vi further strengthens the significance of the detection. The width of this line is 0.7 Å or
90 km s−1 .
5. INTERPRETATION: ONGOING STAR FORMATION IN
CONDENSING FILAMENTS OF INTRACLUSTER GAS
Given the high-ionization energy of the O vi transition
(138.1 eV), it not likely to be due to the same process(es) responsible for the low-ionization Hα emission (Figure 1). The
original detection of O vi in the center of A1795 by Bregman
et al. (2006) was taken as evidence for cooling of the ICM. Alternatively, Sparks et al. (2012) recently invoked an evaporation
scenario to explain the filamentary coronal emission in the core
of the Virgo cluster. In the latter scenario, the hypothesis is that
4
The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
McDonald et al.
aperture to be ∼2–5 Myr and the cooling time to be ∼2 Myr
(assuming ne ∼ 1 cm−3 ). Thus, it is unclear exactly how the
cooling will proceed. Based on the measured O vi λ1038 flux,
and following Edgar & Chevalier (1986), we estimate a cooling
rate of 0.85 ± 0.15 (stat) ± 0.20 (sys) M yr−1 , where the
systematic uncertainty includes expectation for isobaric (1.0 ±
0.2 M yr−1 ) and isochoric (0.7 ± 0.2 M yr−1 ) cooling.
This cooling rate, which probes ∼105.5 K gas, can be
compared to both the X-ray cooling rate and the star formation
rate, which probe the hot (∼107 K) and cold (∼10 K) extremes,
respectively. We estimate the classical cooling rate (ṀX =
2LX μmp /5kT ) within the COS aperture to be ∼1 M yr−1 ,
assuming that, on small (<kiloparsec) scales, the cooling ICM
is similar in morphology (clumpiness) to the UV continuum.
Based on our stellar population modeling (Section 3.2), we
estimate an extinction-corrected star formation rate (assuming
continuous star formation) within the COS aperture of 0.11 ±
0.02 M yr−1 . This suggests that 13+3
−2 % of the gas cooling
through 105.5 K is converted to stars.
Alternatively, some fraction of the O vi emission could come
from a mixing layer, where hot electrons from the ICM penetrate
the cold filaments, following Fabian et al. (2011)—correcting
for this would raise the inferred star formation efficiency.
However, we do not find an excess of optical line emission above
the expectation given the UV continuum level, as is observed
in NGC 1275, suggesting that these effects are small in the
filaments of A1795.
For comparison, Bregman et al. (2006) find a total O viderived cooling rate of 42 ± 9 M yr−1 over the full core
of A1795, while the extinction-corrected star formation rate
(assuming E(B − V ) = 0.1; McDonald et al. 2012b) over the
same area is only 5.2 M yr−1 (McDonald & Veilleux 2009).
This corresponds to an efficiency of forming stars out of the
105.5 K gas of 12+4
−2 %. This is also comparable to the star
formation efficiency found by McDonald et al. (2011) of 14+18
−8 %
based on X-ray spectroscopy and UV photometry.
SUMMARY
We present deep FUV spectroscopy of a cooling filament
in A1795, obtained using the COS on HST. These data allow
us to simultaneously probe the young stellar populations and
the intermediate temperature (105.5 K) gas. We find evidence
for ongoing, in situ star formation, which suggests that the
cool gas in these filaments was not stripped from an infalling
galaxy. The detection of O vi emission suggests that this star
formation is being fueled by condensing intracluster gas, and
that the cooling is proceeding efficiently at high temperatures,
contrary to what is observed on large scales in cluster cores.
We propose a scenario where the two orders of magnitude
disagreement between luminosity-based X-ray cooling rates
and star formation in cluster cores is due to a combination
of globally-inefficient cooling at high temperatures (hot ∼ 0.1;
e.g., AGN feedback) and locally-inefficient star formation at
low temperatures (cold ∼ 0.1).
M.M. acknowledges support provided by NASA through
a Hubble Fellowship grant from STScI and through HST
GO-12992 contract NAS5-26555. S.V. acknowledges support
from a Senior NPP Award held at NASA-GSFC. S.E. acknowledges support from SAO subcontract SV2-82023 under NASA
contract NAS8-03060.
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6. IMPLICATIONS FOR THE COOLING FLOW PROBLEM
Given that cooling flows are found to be ∼1% efficient at
converting the cooling ICM into stars (e.g., O’Dea et al. 2008),
it is important to understand at what temperatures the bulk of
the gas is held up. Assuming that the IMF does indeed follow
Salpeter (1955), we find, for the filaments in A1795 (where
the effects of AGN feedback should be minimized), that the
cooling efficiency at high temperatures is of order 100% (hot ≡
ṀO vi /ṀX ∼ 0.7–1.0), while the star formation efficiency is low
(cold ≡ SFR/ṀO vi ∼ 0.11–0.16). In contrast, Bregman et al.
(2001) found, using O vi observations from FUSE, that hot was
of order 10% over the full cluster core for A1795, A2597, and
Perseus. This suggests that the cooling flow problem may be
divided into two separate inefficiencies.
1. hot ∼ 0.1 : globally inefficient cooling at high temperatures
(107 K → 105 K), due to some large-scale feedback source
(e.g., AGN);
2. cold ∼ 0.1 : locally inefficient cooling at low temperatures
(105 K → stars), manifesting as inefficient star formation.
The latter, which we quantify here and in McDonald et al.
(2011), may be low due to conduction suppressing cooling at
low temperatures. For comparison, star clusters in our Galaxy
have typical star formation efficiencies of ∼8%–30% (Lada &
Lada 2003)—in principle, star formation embedded in a hot
plasma should proceed less efficiently.
5
The Astrophysical Journal Letters, 791:L30 (6pp), 2014 August 20
McDonald et al.
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